Corrosion resistant alloy



Patented June 8, 1937 UNITED STATES CORROSION RESISTANT ALLOY PeterPayson, New York, N. Y.

.No Drawing. Application November 27, 1931,

Serial No. 577,705

4 Claims. (01. 75-128) This invention relates to improved corrosionresistant alloys and particularly to the production of such alloys whichare less susceptible than the usual corrosion resistant alloys tointergranular 5 attack when subjected to elevated temperatures.

In the production of alloys resistant to corrosion from ordinarychemicals, food stuffs, oils, etc., it has been the practice to combinechromium, nickel and iron in varying proportions to give the desiredcharacteristics. An example of an alloy of this type is one containingabout 16 to chromiiun, 7 to 10% nickel, over 45% iron, carbon about 15%,manganese less than .75%, sulphur and phosphorus each less than 035%.Consider- 15 able variation in the proportions of the constituentspresent may be made for the production of alloys of particularcharacteristics and at a suitable cost, a largely sold corrosionresistant alloy being one containing about 18% chromium and 8% nickel. p

In using such alloys for the production of equipment for use in chemicalindustries, it is frequently desirable or necessary to constructlargevessels, tanks, tubes and the like by electric are or gas weldingprocesses or by other heating means. Also, in the use of such equipmentin chemical plants, it is often necessary to subject the alloy toelevated temperatures, for example, 1000 to 1600 F., for a considerableperiod of time. It has been found that when alloys of the above type aresubjected to elevated temperatures for example 1000 to 1600 F., duringawelding operation or while in use, the metal in the vicinity of theweld and the other metal subjected to such elevated temperatures will bestructurally weakened and will be subject to what is frequently idescribed as, weld decay or intergranular corrosion.

For the purposeot testing an alloy to determine 40 whether or not it issubject to intergranular corrosion, it is customary to boil a sample ofthe alloy to be testedin a solution of copper sulphate and sulphuricacid, usually of the strength of about 10 by weight of each. An alloywhich is subject to intergranular corrosion will be, when treated inthis manner, put into a condition in which it can be crumbled betweenthe fingers and will not withstand its own weight upon bending, eventhough the superficial appearance of the material may give no evidenceof corrosion whatsoever. The material between the grains or crystalsconstituting the alloy appears to have been dissolved out without.injury to the. grains themselves. This test is adopted because it isvery drastic in its action and is indicative of the failures that may becaused when an alloy which has been subjected to temperatures between1000 to 1600 F. is exposed to electrolytes.

Various means have been tried for minimizing this intergranularcorrosion, one of which is to use a very low carbon alloy, forexampla-below .07% carbon. Such alloys are, however, diflicult andexpensive to manufacture. Another method sometimes used for lesseningintergranular corrosion is to heat the alloy and to quickly cool it froma temperature above 1850" F. Although this treatment may be carried outwhen dealing with small articles, it is practically impossible, or atleast impracticable, to use it when dealing with large tanks or otherparts of chemical equipment.

In preparing corrosion" resistant alloys to be subjected to welding, ithas heretofore been considered advisable to produce an alloy which wouldbe non-magnetic after welding. This may be accomplished with most alloysof this type by the heat treatment mentioned above. i

It has now been found that by adding certain elements, includingmolybdenum, titanium, vanadium, tungsten and silicon either alone or incombination, in amounts varying for example, between about and 5%, (theamount of added element necessary depending upon the composition of thealloy to which it is added), Delta iron is formed in the alloy andapparently carbon is present mainly within the grains or crystals of theDelta iron rather then along the crystal or grain boundaries or incleavage planes within the crystals, as in the case of alloys previouslyused for thispurpose. Thus when the alloy is heated, tosuch atemperature that the carbides are precipitated they are precipitatedwithin thev grains of Delta iron rather than mainly along the grainboundaries or cleavage planes. Thus,- when my improved alloys aresubjected to temperatures between 1000 to 1600" F'. and then to theabove mentioned treatment in a copper sulphate-sulphuric acid solutionor with other electrolytes, the grain boundaries are not attacked insuch a manner as to seriously' physically weaken the alloys.

The addition of the elements referred to above in the requiredquantities causes the appearance of Delta iron in corrosion resistantsteel previ- 5 To illustrate the application of the above improvementthe'following table is given:

and number Carbon Nickel fi Added element 0. 0. 1 17. 0 None 0. 10 10.00 22. 7 None 4 0. 18 7. 2 21. 9 None 0. 12 8. l0. 3 0.66 vanadium 0. 128. 3 l0. 1 1.88 vanadium 0.18 3.8 22.0 .78v i m 0. 00 9. 7 18. 4 1.00moly bd cum 0. 8. 2 20. 2 3.38 mol spam 0. 17 B. i 20. 0 3.25 mul enum0. ll. 2 23. 7 2.00 silicon 0. 21 ll. 3 23. 1 2.27 silicon 0. 10 8. 320. 5 4.00 tungsten 0. l0 8. l 21. 1 .87 titanium Each of these alloysmay also contain man-; ganese, phosphorus, sulphur or other impuritiesin the amounts normally found in such alloys, for example the amountsreferred toabove. Silicon is ordinarily present in corrosion resistantsteel alloys and would not be considered as an added element unlesspresent in amounts substantially greater than 0.70%.

These alloy steels were subjected to quick cooling from a temperature of1850 to 2100 F. and their magnetic properties were tested after suchcooling. Later they were reheated to within the range of 1000 to 1600 F.and were subsequently subjected to boiling in a copper sulphatesulphuricacid solution, as described above.

Steels 1 to 3 are included as illustrative of corrosion resistant alloysof various chromium and nickel contents. No element was added to thesealloys. Nos. 1 and 2 were nonmagnetic when netic. All of these werefound to be subject to intergranular corrosion.

Steels 4, 5 and .'6 are examples of alloys to which vanadium has beenadded in the proportion given. Steel .4 was non-magnetic after beingcooled from 1850-2100 F. and was susceptible of intergranular attackwhen treated as above described. Steel 5 was magnetic after such coolingand was found to be perfectly resistant sulphate-sulphuric acidsolution. Steel No. 6 was somewhat magnetic and while not perfectlyresistant to chemical action, was highly resistant as compared with No.4.

Steels 7, 8 and 9 illustrate the addition of molybdenum. No. 7 wasnon-magnetic and susceptible of intergranular corrosion. Nos. 8 and 9when treated as described above were magnetic and very highly resistantto intergranular attack.

magnetic and highly resistant. No. 11 was nonmagnetic and was not sohighly resistant to intergranular attack. v v

Steels 12 and 13 are illustrations of the additions of tungsten andtitanium, respectively. Each of these alloys were magnetic after coolingfrom 1850-2100 F. and showed greatly increased resistance tointergranular attack, No.13 being totally resistant even after 420 hoursof boiling in the copper sulphate-sulphuric acid solution.

As illustrated above, the proportions of the added elements must becarefully regulated in order to obtain the full advantage of the im-: 7provement. These proportions will vary with the tested as abovedescribed, while No. 3 was mageven after 400 hours of boiling in thecopper Steels 10 and 11 illustrate the eflect of adding considerablequantities of silicon. No. 10 wasratio of carbon, chromium, nickel andiron in the corrosion resistant alloy and should be such that in theparticular alloy Delta iron will be formed and the alloy will bemagnetic after quickly cooling from 1850 to 2100 I". I

The above illustrations have been given in connection with steelscontaining .09 to .217, carbon, since, as indicated above, certainchromium-nickel-iron alloys containing less than .071, carbon areessentially free from inter-granular attack and may not require theadditional element. The alloys used usually contain at least 0 about 45%iron, about 16 to chromium, about 1 to 20% nickel, and the carbon shouldbe less than about An increase in the carbon content of the steel makesit necessary to add increased amounts of the added element in order toproduce a ferritic or magnetic condition when produce an alloy which ismagnetic upon cooling i from 1850-2100" 1 and which is resistant tointergranular attack.

I have also found that an alloy which is highly resistant tointergranular attack can be produced by adding titanium in ambuntsinsuiiicient to produce a magnetic alloy when cooled quickly from1850-2100 F. The following is an example of such an alloy:

Carbon Silicon Nickel Chromium Titanium Although specific examples havebeen given in describing the invention, it is not intended to therebylimit it to the particular pr portions given, it being apparent that theinvention may be utilized with many difl'erent concentrations ofingredients in the alloys. The terms used in describing the inventionhave been used in their descriptive sense and not as terms of limitationand it is intended that all equivalents of these terms should beincluded within the scope of the claims. In using the termsmolybdenum-silicon element in the appended claims, it is intended toinclude molybdenum, titanium, zirconium, vanadium, tungsten and siliconor their equivalents either alone or in combinations of two or more.

What I claim is:

1. A corrosion resisting alloy steel article, at least a portion ofwhich is exposed in use to temperatures of about 1000 to 1600 I". andthereafter without subsequent heating and quenching from a highertemperature, to liquid media inducing intergranular attack, said articlebeing resistant to said attack, said steel containing: about 16 to 30%chromium, about '1 to 20% nickel. from about .07 to under 25% carbon inproportions to form austenite when cooled quickly from about 1850 to2100 ll, an effective amount up to about 5% molybdenum and an effectiveamount up to about 5% silicon, the

- molybdenum and silicon being present in such 2, A fabricated corrosionresisting alloy steel article; at least a portion of which is exposedduring fabrication to temperatures of about 1000 to 1600 F. and.thereafter without subsequent heating and quenching from a highertemperature, to media inducing intergranular attack, said articlebeing'resistant to said 'attack, said steel containing: about 16 to 30%chromium, about 7 to 20% nickel, from about .07 to under .25% carbon inproportions to form austenite when cooled quickly from about 1850 to2100 R, an effective amount up to about 5% molybdenum and an effectiveamount up to about 5% silicon, the molybdenum and silicon being presentin such proportions as to form a resulting alloy steel containing deltaferrite.

3. A corrosion resisting alloy steel article, at least a portion ofwhich is exposed in use to temperatures of about 1000 to 1600 F. andthere- 20 after without subsequent *heating and quenching from a highertemperature, to liquid media inducing intergranular attach, said articlebeing resistant to said attack, said steel containing: about 16 to 30%chromium, about 7 to 20% nickel, from about .07 to under .25% carbon inproportions to form austenite when cooled quickly from about 1850 to2100 F., and an effective amount up to about 5% molybdenum; themolybdenum being present in such proportion as to form a.resulting alloysteel containing delta ferrite.

4. A fabricated corrosion resisting alloy steel article, at least aportion of which is exposed during fabrication to temperatures of about1000 to 1600 F. and thereafter without subsequent heating and quenchingfrom a higher temperature, to media inducing intergranular attack, saidarticle being resistant to said attack, said steel containing: about 16to 30% chromium, about 7 to 20% nickel, from about .07 to under .25%carbon in proportions to form austenite when cooled quickly from about1850 to 2100 F., and an effective amount up to about 5% molybdenum, themolybdenum being present in such proportion as to form a. resultingalloy steel containing delta ferrite.

' PETER PAYSON.

